Refactor and simplify recipe DSL evaluation usages:

Remove most of recipe evaluation code in Recipe class, and rely on
Recipe::DSL to get evaluated recipes.

* Remove Recipe#evaluate call from CLI, rely on
  Recipe.evaluate_from_file to get the evaluated recipe;
* Implement Recipe.evaluate_from_file(filepath, env);
* Implement Recipe::DSL.evaluate(code, env);
* Remove code and filepath accessor on Recipe;
* Remove Recipe.from_file and Recipe#evaluate methods;
* Move task evaluations in Recipe::DSL#evaluate;
* Modify Recipe constructor so that it accepts tasks as argument.

lib/producer/core/cli.rb

@@ -12,7 +12,6 @@ module Producer
 
       def run!
         check_arguments!
-        recipe.evaluate(env)
         worker.process recipe.tasks
       end
 
@@ -25,7 +24,7 @@ module Producer
       end
 
       def recipe
-        @recipe ||= Recipe.from_file(@arguments.first)
+        @recipe ||= Recipe.evaluate_from_file(@arguments.first, env)
       end
 
       def worker